<communications, protocol> (HDSL) A form of Digital Subscriber Line, providing T1 or E1 connections over two or three twisted-pair copper lines, respectively. Unlike most other forms of DSL HDSL is not a typical consumer service, it's used mostly to replace traditional T1/E1 connections, such as connecting PBXes to telco offices. The advantage of HDSL over the Alternate Mark Inversion line coding scheme traditionally used on T1/E1 lines is that it requires about an order of magnitude lower bandwidth to carry the same traffic.